Cracking process



Nov.- 19, 1935. A, MCG. woon v 2,021,761

CRACKING PROCESS- Filed oef. 5. 1951 VIA" gmc/who@ Patented Nov. 19, 1935 ATENT QFFICEA CRACKING PROCESS Andrew McGregor Wood, Miri, Sarawak, as-

signonby mesne assignments, to Universal Oil Products Company, Chicago, Ill., a corporation of South Dakota pplication October 5, 1931, Serial No. 566,853

3 Claims. (ICI. 196-66) This invention relates to the process of cracking consisting inthe conversion of heavier hyy drocarbons into lighter hydrocarbons usually of incondensible gases and are all products of overcracking, brought about by too extremeheating of part of the cracking feed or maintaining such heating conditions for too long` a time. However, these heat and timefactors are both necessary in order to effectively treat all of the constituents .of the cracking feed because it is composed of numerous hydrocarbon 'oils in intimate mixture and the best that can be done is to -clioose operating conditions such that the greatest number of hydrocarbons in the feed will be cracked to the desired fraction. In choosing-such conditions there will always remain some hydrocarbons that are little aifected and some of the more easily decomposed oils that have cracked into residues and light incondensible gases. Those that are not cracked can be retreated but the others are lost as far as gasoline yield is concerned. Be-` sides constituting a material waste these products are a great nuisance .in operating a cracking apparatus, particularly the carbonaceous residues, which deposit inthe system and necessitate frequent cleanings and renewal of equipment.

I have found that these diiculties will be overcome or greatly reduced by the combined release of light fractions as soon as formed and injection of incondensible gas at frequent intervals during the passing of the oil through the heater; this Will maintain conditions of chemical and thermodynamical equilibrium favorable to the formation of gasoline and unfavorable to the formation of uncondensible gases and coke. The removal of the gasoline vapors as soon as formed prevents their overcracking, while the replenishing of the uncondensible gas, removed together with the gasoline vapors, prevents the formation of new gas to fill the void in the equilibrium system caused by the removal.

Hereafter, I will describe one method of carwith the liquid level, inside the chamber.

rying my invention into` practice. It will be understoodthat this is solely as example and that other means can be devised to carry the principle of my invention into eifect.

The drawing illustrates diagrammatically an 5 embodiment of an apparatus to carry out my process. Fig. l represents a sectional elevation of a coil heating furnace of any well known type for illustrating the use of my invention. Fig. 2 shows a detailed view of a gas release chamber. l partly in section, as used in the process.

In the drawing I represents a furnace showing the heating coils 3 therein. The pipe 2 is a supply line carrying the feed to the furnace coils and is controlled by the valve I6. At intervals l5 throughout the path of the coil 3 through the furnace are located the gas release chambers 5, 6 and 'I outside of the heater walls. 'I'he transfer line 4 extends from the coil 3 on its final exit from the furnace to an evaporator or expansion chamber (not shown) of any well known type. The control valve 20 in the transfer line regulates the pressure in the heater. The line 8 from chamber 5, joined by line 9 from chamber 6, and linev I0 from chamber 1, passes to any 25 evaporator, expansion chamber or dephlegmator used in connection with the process. These lines have the control valves as follows: I1 in line 8,

I8 in line 9 and I9 in line.l0. The line represented 'by II comes from a gas container under 30 5, 6 and 'I respectively, as will be described in detail in considering Fig. 2, which shows an .enlarged view of the gas release chamber 5' and is identical in operation and construction with chambers 6 and 1. This chamber is located at a return bend of coil 3 outside of the furnace walls. It consists of a oat 2l held by'the rod 45 28 to the pivot 29 and capable of rising vertically At its highest pointl the valve 30 is positioned in. the seat 3| so as to close the'vapor exit tube 8. The gas tube I3 enters the bottom of the chamber 5 50 and extends into the coil 3 on its chamber exit side. Itis of materially smaller diameter than the coil, usually only one-fourth to one-eighth o f such size, and extends to a point where the ilow of oil thru the coil will prevent the return of 4any gas against its current which might otherwise escape back and lodge in the vapor release ,it has also been found advantageous to position the nozzle 32 such that it lies against one of the coil walls and at an angle with the coil axis, such that the expulsion of gas therefrom gives the oil within the coil a rotary motion and ir.- creased turbulence.

My improved furnace arrangement can be used with any of the well known cracking plants employing vapor separating, dephlegmating and condensing apparatus with a P. D. receiver. In the operation of my invention with such a plant it is also necessary to have some means of removing the incondensible gases from the pressure distillate and storing them under pressure in a suitable container. The cracking feed is then brought to the furnace through the line 2. As heating is started in the coil 3 a certain amount of incondensible gas from the line I2 is injected into the oil. It is a well known fact that when oil is being cracked an equilibrium varying with time and temperature is established in which there are certain amounts of all of the series of hydrocarbons present and that if one of them is separately added the equilibrium will be satisfied without the formation of so much of lthat hydrocarbon from the oil undergoing cracking. In this manner the injection of these incondensible gases greatly retards the formation of such in the cracking feed. Due to this inhibiting l effectl on the formation of the lightest gases such as methane, ethane, propane, etc., the amount of those forming next higher in the series, such as the gasoline like fractions, will be proportionately increased. As these fractions form they bubble thru the oil with the injected incondensible gases producing a turbulent gas-oil mixture, but before further, or overcracking of these products can take place, they have reached the chamber 5. Here the vapors bubble up thru the turbulent liquid and are caught in the vapor trap or zone above the liquid, being the highest portion of chamber 5. As they form in this zone the liquid level is depressed until the float 21 drops, thereby releasing the valve`30 from its seat as shown in Fig. 2. The vapor will then escape thru the valve and be passed to the transfer line 8. This process will go on intermittently,

the valve being opened whenever sufficient vapor collects, or remain more or less continuously open if cracking and vapor formation go on fast enough in the preceding section of the coil. The most easily cracking fractions being removed,

the oil now contains only those which requiremore treatment as it continues its path thru the coil past chamber 5. More incondensible gas is immediately injected from the line I3 thru nozzle 32 to replace that lost at chamber 5 and the process continues as before. The next cracked fraction of hydrocarbons in the feed are removed at chamber 6, operating in thesame manner as chamber 5, the vapor passing out thru line 9 past the valve I8 and into the transfer line 8. The content of incondensible gas is again immediately replenished from the valve controlled line I4, and the reaction goes on as before, this time with the still more resistant hydrocarbons. The

next cracked oils are mostly removed as vapor at chamber 1 thru the line I0, past the regulating valve I9 to the line 8. The incondensible gas content is again renewed from the line I5 and the remaining hardest cracking oils are treated till their exit from the furnace. They here pass into the transfer line 4 past the pressure regulating valve 20 and on into the evaporator or expansion chamber. The rest of the process is handled according to the usual methods of operation.

By way of illustrating the operation of my process, the following example is given: Ventura fuel oil was fed into the furnace coil at around 350 pounds pressure. At chamber 5 with removal of cracked products the oil had reached a temperature of about 850 F. and the pressure was around 325 pounds. At chamber 6 on removal of more cracked products the temperature was around S50-875 F. and the pressure had dropped off to around 295 pounds. At chamber 7 the temperature was still about the same while the pressure had dropped to around 260 pounds. The remaining oil vapor mixture was still around S50-875 F. on leaving the furnace thru line 4, and had a pressure around 220 pounds.

The oil is continually absorbing more heat in its path thru the furnace but the temperature remains about constant after nearly reaching its maximum at chamber 5. This is probably due to the continuous loss of latent heat in the oil due to the formation and removal of new cracked compounds.

Theresults shown in the following examples were obtained from two cracking furnaces each having 104 tubes of 30 ft. length and 4 inch inside diameter. The results in the columns I were obtained from the furnace equipped to operate according to my invention with gas injection and release of crackedV products, while the results in the columns IIl were from the other furnace operated in the usual manner. The following table shows the results obtained from a deep, a medium, and a light cracking, of Ventura fuel oil.

The incondensible gas for injection intothe coil is supplied by any suitable compression means, necessarily having a pressure greater than that of the oil in thel furnace coil. Due to the accelerated cracking rate of the oil as it progresses thru the furnace, each injection of incondensible gas must usually be somewhat larger than the one just previous, to maintain equilibrium in each of the `successive zones in the heating coil. The exact amounts of gas to 'l be' injected are readily determined by a skilled operator. Usually a certain valve setting (on each of the valves 2|, 22, 23 and 24) is found at which lthe system functions most efficiently and the gases are continuously injected. However, this setting varies considerably with the gas pressure used, the heating coil pressure encountered,

the temperature of the oil, and the nature of the oil undergoing cracking. y

It will be noted in the above table that in the light cracking of the oil the production of gas is practically nil and'it is consequently necessary to obtain a gas for injection from an outside source. It may also be often desirable to use gas from an outside source for injection into the oil during deeper cracking operations, as my process is in no way limited to the use of gas produced in the cracking system. Any cracked gas or natural hydrocarbon gas from any source is suitable for injection.

The temperatures and pressures noted in the above illustration of my process are, of course, subject to awide variation, depending on the type of feed being cracked in the furnace, to some extent on the nature of the products vdesired, and the degree of heat applied by the f urnace burner 2E, thru the circulating iiue gases. For,eXamp1e, if kerosene or gas oil is being cracked the system functions with increased efiiciency at materially higher temperatures and pressures.

By treating the oil according to my processthe very utmost toward cracking eiiiciency is attained. The pressure distillate yield is high as very few waste products are formed. There is very little coking in the heater because overcraclcing in the coils is practically eliminated by the successive removal of cracked products. The periodic injection of the compressed gas from the lines l2, i3, M and I also has a tendency to prevent the deposition of small amounts of carbon that may be formed in the coils, by increasing the turbulence of the oil as it passes therethru. This turbulent flow of the oil is also of considerable help in cracking as it makes pos` v'I further do not limit myself to the particular Usually notl design of the vapor release chamber shown in, the drawingsas any device accomplishing like results may be employed.

lI claim as my invention:

1. In theheating of hydrocarbon oils to crack- 5 I ing temperature while passing in a restricted stream through an elongated heating coil disposed in a furnace, the improvement which comprises separating vapors from the oil stream at V spaced points in its pathvof travel through ,the heating coil, and injecting incondensible hydrocarbon gas into the oil stream at points immediately following the points v'of vapor separation, the gas being injected in the direction ot iiow of the oil stream through thev heating coil. f

2. In the heating of Ahydrocarbon oils to cracking temperature While passing in a restricted stream through an elongated heating coil, the improvement which comprises introducing incondensible hydrocarbon gas to the oil as it enters the heating coil, separating vapors and gases from the oil 'stream at an intermediate point in its Ip ath of travel through the heating coil, injecting. additional incondensible hydrocarbon gas into the oii stream in the direction of ow of Vthe latter throughv the heating coil immediately following said/point of vapor and gas separation, and continuing the passage of the' oil stream through theA remainder of the heating coil. I"

3. In the heatingofhydrocarbon oils to crack-A ing temperature while passing in a restricted stream through-an. elongated heating coil, the improvement which comprises introducing in2 condensible hydrocarbon vgas to the oil as it 35 enters' the heating coil, separating vapors and' gases from 'the ,oil stream at an intermediate point in its path of travel through the heating coil, injecting a larger amount ofincondensible hydrocarbon gas into the oil stream in the direc- 40 4tion of owof the latter through the heating coil immediately following said point of vapor and gas separation, and continuing the passage of the oil stream through the remainder of the heating coil.

ANDREW MCGREGOR WOOD. 

